This invention relates to processes for producing silicon carbide ceramic micro tubes (SiC micro tube) having outside diameters of from several to several hundred microns and wall thicknesses of several microns. The invention further relates to processes for producing SiC micro tube while permitting their wall thickness to be controlled at a desired value within the range of 2-20 μm.
SiC is a material of high resistance, high strength and low reactivity and SiC fiber synthesized from silicon-based polymers has already been commercialized whereas active research efforts are being made to use the SiC fiber as a reinforcement and produce SiC fiber reinforced SiC composite materials. The current method for producing SiC micro tube comprises the following steps:    1. spinning a silicon polymer to make a fiber having a diameter of several tens of microns;    2. applying an ionizing radiation to the fiber in the air at room temperature such that the surface part of the fiber is selectively oxidized;    3. heat treating the fiber in an inert gas to crosslink the oxidized layer so that it becomes insoluble in a solvent;    4. extracting the uncrosslinked core part of the fiber with an organic solvent to make it hollow; and    5. heat treating the resulting polymer tube in an inert gas at 1000° C. and above so that it becomes ceramic.
While it is possible to control the wall thickness of the tube by changing the dose rate and oxygen partial pressure employed in irradiation, the smallest wall thickness that can be achieved for tubes presently produced by irradiation with electron beams at room temperature is about 10 microns [U.S. Pat. No. 6,780,370; Development of Silicon Carbide Micro Tube from Precursor Polymer by Radiation Oxidation, Masaki Sugimoto and others, Key Eng. Mater., 247, 133-136 (2003); “Denshisen no shosha koka de jitsugen shita tanka keiso seramikkus maikuro chubu (Silicon Carbide Ceramic Micro tubes Realized by the Irradiating Effect of Electron Beams)”, Masato Yoshikawa and Masaki Sugimoto, ConverTec, 377, pp. 56-60 (2004)].
The method under consideration is capable of controlling the wall thickness of the SiC micro tube by changing the dose rate and oxygen partial pressure employed in irradiation; however, in the irradiation with electron beams at room temperature, the energy of the electron beams applied is absorbed by the fiber and its temperature is elevated, making it difficult to control the wall thickness of the tube at 10 μm and below. Therefore, it is impossible to make a hollow tube from the industrially mass-produced 15 μmφ SiC fiber with its wall thickness controlled to any value that suits an intended use.
The present invention provides a process for producing a SiC micro tube from a silicon-based polymer fiber with its wall thickness controlled to a desired value within the range of 2 to 20 μm either by starting from a polymer blend of two silicon-based polymers, polycarbosilane and polyvinylsilane, or by cooling the silicon-based polymer fiber during irradiation with electron beams. The use of the polymer blend as the starting material has the additional advantage of simplifying the conventional process by eliminating the step of heat treatment.